1. Field of the Invention
This invention resides in the field of laboratory apparatus for biological and biochemical laboratories, and particularly to apparatus for cell counting.
2. Description of the Prior Art
Cell counting in suspensions of blood cells, bacteria, and biological cells in general is a procedure commonly used in clinical and research laboratories. Measurement of the number of cells per unit volume of the suspension by cell counting is valuable in assessing such qualities as biological function and activity as well as cell viability and growth. Cell counting is commonly performed on a hemocytometer, which utilizes a microscope slide to hold the suspension and a microscopic viewer to allow viewer observation of the slide for manual counting, either directly or from an image of the slide. To count cells in a suspension, a drop of the suspension is placed in the slide and covered by a second slide or any thin transparent cover slip. The gap (also referred to as the “chamber”) between the slide and the cover slip is typically on the order of 100 microns. A grid on the underside of the cover slip delineates an area for counting, and knowledge of both the area and the depth of the gap allows the viewer to determine the cell count per unit volume. The grid enables the viewer to select a portion of the grid whose area is small relative to the dimensions of the slide, thereby enabling the user to count cells with relative ease and quickness and with minimal risk of error. A critical feature of the slide and cover slip combination is therefore the distance between them, and another is the evenness of the distribution of the cells across the width of the slide. If the distance is not uniform or the cells are not distributed evenly, the concentration calculated from the selected area can differ from the concentration in the suspension as a whole.
Conventional means of placing the suspension in the chamber of a cell counting slide involve the use of a pipette to introduce the suspension to one side of the chamber through a notch or similar opening in the cover sheet. The suspension then flows from the notch into the chamber. Examples of disclosures of slides of this type and their use are found in Tolles, W., U.S. Pat. No. 4,171,866, issued Oct. 23, 1979, and Fisch, H., U.S. Pat. No. 5,349,436, issued Sep. 20, 1994. The result of this procedure however is often an uneven distribution of the cells, with the concentration being relatively high in areas to the sides of the notch than in areas near the center of the notch due to differences in the flow direction and flow rate of the suspension at different points within the chamber. The concentration of cells at the center of the chamber, which is typically where the counting takes place, is therefore different (most often lower) than the concentration of the suspension as a whole. The present invention addresses this problem.